#include<vector>
#include<string>
using namespace std;
class Solution {
public:
    int numDecodings(string s) {
        int n = s.size();
        vector<int> dp(n);
        // if(s[0] == '0')return 0;
        if (s[0] == '0')
            return 0;
        dp[0] = 1;
        if (n == 1)
            return dp[0];

        if (s[1] >= '1' && s[1] <= '9')
            dp[1] += dp[0];
        int t = (s[0] - '0') * 10 + (s[1] - '0');
        if (t <= 26 && t >= 10)
            dp[1] += 1;

        for (int i = 2; i < n; i++) {
            if (s[i] >= '1' && s[i] <= '9') {
                dp[i] += dp[i - 1];
            }
            int t = (s[i - 1] - '0') * 10 + (s[i] - '0');
            if (t <= 26 && t >= 10) {
                dp[i] += dp[i - 2];
            }
        }
        return dp[n - 1];
    }
};